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          <span data-v-4e82dffc>2021-9-1 - </span>
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        </a></span></div></div> <div data-v-1156296a><main class="page"><section><div class="page-title"><h1 class="title">Go语言基础之包(package)</h1> <div data-v-1ff7123e><i class="iconfont reco-account" data-v-1ff7123e><span data-v-1ff7123e>小狐</span></i> <i class="iconfont reco-date" data-v-1ff7123e><span data-v-1ff7123e>9/2/2021</span></i> <!----> <i class="tags iconfont reco-tag" data-v-1ff7123e><span class="tag-item" data-v-1ff7123e>GO</span></i></div></div> <div class="theme-reco-content content__default"><h1 id="go语言基础之包"><a href="#go语言基础之包" class="header-anchor">#</a> Go语言基础之包</h1> <p>在工程化的Go语言开发项目中，Go语言的源码复用是建立在包（package）基础之上的。本文介绍了Go语言中如何定义包、如何导出包的内容及如何导入其他包。</p> <h1 id="go语言的包-package"><a href="#go语言的包-package" class="header-anchor">#</a> Go语言的包（package）</h1> <h2 id="包介绍"><a href="#包介绍" class="header-anchor">#</a> 包介绍</h2> <p><code>包（package）</code>是多个Go源码的集合，是一种高级的代码复用方案，Go语言为我们提供了很多内置包，如<code>fmt</code>、<code>os</code>、<code>io</code>等。</p> <h2 id="定义包"><a href="#定义包" class="header-anchor">#</a> 定义包</h2> <p>我们还可以根据自己的需要创建自己的包。一个包可以简单理解为一个存放<code>.go</code>文件的文件夹。 该文件夹下面的所有go文件都要在代码的第一行添加如下代码，声明该文件归属的包。</p> <div class="language-go line-numbers-mode"><pre class="language-go"><code><span class="token keyword">package</span> 包名
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>注意事项：</p> <ul><li>一个文件夹下面直接包含的文件只能归属一个<code>package</code>，同样一个<code>package</code>的文件不能在多个文件夹下。</li> <li>包名可以不和文件夹的名字一样，包名不能包含 <code>-</code> 符号。</li> <li>包名为<code>main</code>的包为应用程序的入口包，这种包编译后会得到一个可执行文件，而编译不包含<code>main</code>包的源代码则不会得到可执行文件。</li></ul> <h2 id="可见性"><a href="#可见性" class="header-anchor">#</a> 可见性</h2> <p>如果想在一个包中引用另外一个包里的标识符（如变量、常量、类型、函数等）时，该标识符必须是对外可见的（public）。在Go语言中只需要将标识符的首字母大写就可以让标识符对外可见了。</p> <p>举个例子， 我们定义一个包名为<code>pkg2</code>的包，代码如下：</p> <div class="language-go line-numbers-mode"><pre class="language-go"><code><span class="token keyword">package</span> pkg2

<span class="token keyword">import</span> <span class="token string">&quot;fmt&quot;</span>

<span class="token comment">// 包变量可见性</span>

<span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">100</span> <span class="token comment">// 首字母小写，外部包不可见，只能在当前包内使用</span>

<span class="token comment">// 首字母大写外部包可见，可在其他包中使用</span>
<span class="token keyword">const</span> Mode <span class="token operator">=</span> <span class="token number">1</span>

<span class="token keyword">type</span> person <span class="token keyword">struct</span> <span class="token punctuation">{</span> <span class="token comment">// 首字母小写，外部包不可见，只能在当前包内使用</span>
	name <span class="token builtin">string</span>
<span class="token punctuation">}</span>

<span class="token comment">// 首字母大写，外部包可见，可在其他包中使用</span>
<span class="token keyword">func</span> <span class="token function">Add</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> y <span class="token builtin">int</span><span class="token punctuation">)</span> <span class="token builtin">int</span> <span class="token punctuation">{</span>
	<span class="token keyword">return</span> x <span class="token operator">+</span> y
<span class="token punctuation">}</span>

<span class="token keyword">func</span> <span class="token function">age</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 首字母小写，外部包不可见，只能在当前包内使用</span>
	<span class="token keyword">var</span> Age <span class="token operator">=</span> <span class="token number">18</span> <span class="token comment">// 函数局部变量，外部包不可见，只能在当前函数内使用</span>
	fmt<span class="token punctuation">.</span><span class="token function">Println</span><span class="token punctuation">(</span>Age<span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br></div></div><p>结构体中的字段名和接口中的方法名如果首字母都是大写，外部包可以访问这些字段和方法。例如：</p> <div class="language-go line-numbers-mode"><pre class="language-go"><code><span class="token keyword">type</span> Student <span class="token keyword">struct</span> <span class="token punctuation">{</span>
	Name  <span class="token builtin">string</span> <span class="token comment">//可在包外访问的方法</span>
	class <span class="token builtin">string</span> <span class="token comment">//仅限包内访问的字段</span>
<span class="token punctuation">}</span>

<span class="token keyword">type</span> Payer <span class="token keyword">interface</span> <span class="token punctuation">{</span>
	<span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">//仅限包内访问的方法</span>
	<span class="token function">Pay</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">//可在包外访问的方法</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><h2 id="包的导入"><a href="#包的导入" class="header-anchor">#</a> 包的导入</h2> <p>要在代码中引用其他包的内容，需要使用<code>import</code>关键字导入使用的包。具体语法如下:</p> <div class="language-go line-numbers-mode"><pre class="language-go"><code><span class="token keyword">import</span> <span class="token string">&quot;包的路径&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>注意事项：</p> <ul><li>import导入语句通常放在文件开头包声明语句的下面。</li> <li>导入的包名需要使用双引号包裹起来。</li> <li>包名是从<code>$GOPATH/src/</code>后开始计算的，使用<code>/</code>进行路径分隔。</li> <li>Go语言中禁止循环导入包。</li></ul> <h3 id="单行导入"><a href="#单行导入" class="header-anchor">#</a> 单行导入</h3> <p>单行导入的格式如下：</p> <div class="language-go line-numbers-mode"><pre class="language-go"><code><span class="token keyword">import</span> <span class="token string">&quot;包1&quot;</span>
<span class="token keyword">import</span> <span class="token string">&quot;包2&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="多行导入"><a href="#多行导入" class="header-anchor">#</a> 多行导入</h3> <p>多行导入的格式如下：</p> <div class="language-go line-numbers-mode"><pre class="language-go"><code><span class="token keyword">import</span> <span class="token punctuation">(</span>
    <span class="token string">&quot;包1&quot;</span>
    <span class="token string">&quot;包2&quot;</span>
<span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h2 id="自定义包名"><a href="#自定义包名" class="header-anchor">#</a> 自定义包名</h2> <p>在导入包名的时候，我们还可以为导入的包设置别名。通常用于导入的包名太长或者导入的包名冲突的情况。具体语法格式如下：</p> <div class="language-go line-numbers-mode"><pre class="language-go"><code><span class="token keyword">import</span> 别名 <span class="token string">&quot;包的路径&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>单行导入方式定义别名：</p> <div class="language-go line-numbers-mode"><pre class="language-go"><code><span class="token keyword">import</span> <span class="token string">&quot;fmt&quot;</span>
<span class="token keyword">import</span> m <span class="token string">&quot;github.com/Q1mi/studygo/pkg_test&quot;</span>

<span class="token keyword">func</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
	fmt<span class="token punctuation">.</span><span class="token function">Println</span><span class="token punctuation">(</span>m<span class="token punctuation">.</span><span class="token function">Add</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">,</span> <span class="token number">200</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
	fmt<span class="token punctuation">.</span><span class="token function">Println</span><span class="token punctuation">(</span>m<span class="token punctuation">.</span>Mode<span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>多行导入方式定义别名：</p> <div class="language-go line-numbers-mode"><pre class="language-go"><code><span class="token keyword">import</span> <span class="token punctuation">(</span>
    <span class="token string">&quot;fmt&quot;</span>
    m <span class="token string">&quot;github.com/Q1mi/studygo/pkg_test&quot;</span>
 <span class="token punctuation">)</span>

<span class="token keyword">func</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
	fmt<span class="token punctuation">.</span><span class="token function">Println</span><span class="token punctuation">(</span>m<span class="token punctuation">.</span><span class="token function">Add</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">,</span> <span class="token number">200</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
	fmt<span class="token punctuation">.</span><span class="token function">Println</span><span class="token punctuation">(</span>m<span class="token punctuation">.</span>Mode<span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><h2 id="匿名导入包"><a href="#匿名导入包" class="header-anchor">#</a> 匿名导入包</h2> <p>如果只希望导入包，而不使用包内部的数据时，可以使用匿名导入包。具体的格式如下：</p> <div class="language-go line-numbers-mode"><pre class="language-go"><code><span class="token keyword">import</span> <span class="token boolean">_</span> <span class="token string">&quot;包的路径&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>匿名导入的包与其他方式导入的包一样都会被编译到可执行文件中。</p> <h2 id="init-初始化函数"><a href="#init-初始化函数" class="header-anchor">#</a> init()初始化函数</h2> <h3 id="init-函数介绍"><a href="#init-函数介绍" class="header-anchor">#</a> init()函数介绍</h3> <p>在Go语言程序执行时导入包语句会自动触发包内部<code>init()</code>函数的调用。需要注意的是： <code>init()</code>函数没有参数也没有返回值。 <code>init()</code>函数在程序运行时自动被调用执行，不能在代码中主动调用它。</p> <p>包初始化执行的顺序如下图所示：<img src="https://www.liwenzhou.com/images/Go/package/init01.png" alt="包中的init()执行时机"></p> <h3 id="init-函数执行顺序"><a href="#init-函数执行顺序" class="header-anchor">#</a> init()函数执行顺序</h3> <p>Go语言包会从<code>main</code>包开始检查其导入的所有包，每个包中又可能导入了其他的包。Go编译器由此构建出一个树状的包引用关系，再根据引用顺序决定编译顺序，依次编译这些包的代码。</p> <p>在运行时，被最后导入的包会最先初始化并调用其<code>init()</code>函数， 如下图示：<img src="https://www.liwenzhou.com/images/Go/package/init02.png" alt="包之间的init()执行顺序"></p></div></section> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">2021-9-2: </span> <span class="time">9/3/2021, 6:20:18 PM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev"><a href="/blogs/GO/go/26_go_reflect.html" class="prev">
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